A. Field of the Invention
This invention relates to the field of load cells, and more particularly to parallel beam load cells.
B. Prior Art
It has long been well known to measure forces by utilizing strain gage elements bonded to an elastic beam which is loaded in bending by the unknown force. The use of two parallel beams rigidly interconnected at their ends in order to make the cell insensitive to the point of application of load has also been described. The insensitivity of a parallel beam load cell with respect to the location of the load follows from the fact that the loaded end of the beams, by the very nature of the parallelogram configuration, is constrained to move in a straight line. A simple stress analysis of a parallel beam load cell shows that the bending moments arising from the application of load not directly in line with the mid point of the beams (either longitudinally or transversely) are not resisted by the elastic restraint of the beams in the direction of the application of force, as is the direct force of the load, but the bending moments are rather resisted by axial tensile and compressive stresses, and/or by transverse bending stresses in the beams. Fortunately, the strain gage elements used can be positioned, and electrically connected, in such a way that the resistance changes due to axial beam stresses (caused by longitudinal bending moments) cancel, and do not alter the cell output. The effects of the transverse bending moments may be eliminated also by positioning the strain gage elements symmetrically with respect to the neutral axes of the beams (transversely) so that beam bending in the transverse direction does not cause the net resistance of the gages to change. Thus, despite varying strains in the beams caused by the shifting position of the load, the load cell output can be made to be responsive only to the magnitude of the applied load.
It has been found, however, that this fortuitous result depends upon maintaining a high degree of symmetry and homogeneity of the structure, and the strain gage elements bonded thereto. To the extent that there are asymmetries and inhomogeneities, the load cell will actually have a component of output which is a function of the point at which the load is applied, and this is a problem which must be faced when making high accuracy measurements.
The problem has been considered by inventors in the prior art by, e.g., Lockery, U.S. Pat. No. 3,576,128 and Farr, U.S. Patent No. 3,927,560. Lockery discloses an electrical method of compensating for load position errors due to load position changes in the axial direction with respect to the loaded beams, i.e., compensation for longitudinal moments about a transverse axis. Lockery utilizes the principle of desensitizing two of the strain gage elements of a four element bridge until the resistance changes of these two elements as a function of load position just offsets the resistance changes of the other two strain gage elements, so that no bridge output due to load position change appears.
Farr discloses a mechanical method of equalizing the resistance changes. Farr alters the cross sectional area of both beams in the parallelogram structure such that two strain gage elements which are subject to opposite stress changes due to load position changes (about the transverse axis) are made more sensitive to load position changes, to become just equal and opposite to the sensitivity of the other two strain gage elements, as a function of load position.
Neither of these patentees, nor others, have discussed the problem of load position sensitivity of load cells with respect to position changes transverse to the beam axes, i.e., moments about the longitudinal axis of the load cell.